1. Field of the Invention
The present invention relates to a wideangle zoom lens device suitable for use in video cameras, SV (Still Video) cameras and so forth. More particularly, the present invention is concerned with a wideangle zoom lens that comprises four lens groups including a lens group which is on the object-side or leading end of the lens that has a negative refractive power. The four lens groups are suitably constructed and arrnaged to attain a variable power of 3.0 and an F number of 1.7, while reducing the overall size of the zoom lens.
2. Description of the Related Art
Hitherto, a zoom lens generally referred to as "negative-lead type zoom lens" has been known in which a lens group having a negative refractive power is provided on the object-side or leading end of device having a plurality of lens groups. This type of zoom lens has been broadly used as wideangle zoom lens devices having a view angle greater than 70.degree., because it is rather easy to obtain a large view angle with this type of zoom lens. For instance, U.S. Pat. No. 3,771,853 and Japanese Examined Patent Publication (Kokoku) No. 61-55094 discloses a zoom lens device of a type having a first lens group disposed on the object-side or leading end of the zoom lens device, a second lens group of a positive refractive power, a third lens group of a negative refractive power and a fourth lens group disposed on the imaging-side or trailing end of the lens device, wherein variation from the power from the wideangle end to the telephoto end is effected by moving the first lens group toward the image plane and the second and fourth lens groups toward the object, while fixing or suitably moving the third lens group.
Japanese Examined Patent Publication (Kokoku) No. 61-55094 (Japanese Unexamined Patent Publication No. 57-163213) proposes a zoom lens device which employs a 4-group lens system of negative, positive, negative and positive power, from the object side, wherein the third group is fixed so as to attain an F number of 4.0 or so, with a back focus of about 1.12 l, where l represents the length of the diagonal line of the photographing area.
In general, however, a negative-lead type zoom lens requires a careful consideration as to the refractive powers of the lens groups, in order to provide superior optical performance. The zoom lenses seek to attain a large view angle of 70.degree. or greater and a large aperture ratio of 1.7 or so, while maintaining a required value of back focus, although widening of view angle is rather easy to attain with this type of zoom lens. Inappropriate arrangement of the refractive powers of the lens groups, as well as inadequate element arrangements in lens groups, causes aberration to fluctuate. This is largely in response to a zooming operation, regardless of an increase in the number of the lenses, thereby making it difficult to attain superior optical performance over the entire range of power variation.
For instance, although a wider view angle and longer back focus are obtainable by an increase in the negative refractive power of the first lens group, an unduly large negative refractive power of this lens group causes the aberration to vary significantly in response to a zooming operation.
In general, a single-lens reflex still-photography camera requires a predetermined back focus length in order to preserve a space for accommodating a quick return mirror. A video camera also requires that a predetermined back focus be maintained, so as to provide room for installation of various components such as a CCD, a cover glass, an optical filter and so forth. It is possible to obtain a large back focus by designing the whole lens system as an inverted telephoto lens system. Such a measure, however, makes it difficult to obtain superior optical performance over the entire range of variable power, due to the asymmetry of the whole lens system.